A fuel cell is a novel electric power production system that directly converts chemical energy generated by the electrochemical reaction between fuel (hydrogen or methanol) and an oxidizing agent (oxygen or air) into electrical energy. The fuel cell has attracted considerable attention as a next-generation energy source by virtue of the high energy efficiency and the low contaminant discharge, i.e., the environmentally friendly characteristics, and much research on the fuel cell has been carried out.
Based on the kinds of electrolytes used, fuel cells are classified into a phosphoric acid fuel cell, an alkaline fuel cell, a polymer electrolyte fuel cell, a molten carbonate fuel cell, and a solid oxide fuel cell. Among them, the polymer electrolyte fuel cell is classified as a proton exchange membrane fuel cell using hydrogen gas as fuel or a direct methanol fuel cell in which liquid-phase methanol, as direct fuel, is supplied to an anode.
The polymer electrolyte fuel cell is in the spotlight as a portable power supply unit, a power supply unit for vehicles, or a power supply unit for home use by virtue of low operating temperature of 100° C. or less, elimination of leakage problems due to the use of a solid electrolyte, rapid starting and response characteristics, and excellent durability. Especially, the direct methanol fuel cell has a simple fuel supply system, and the overall structure of the direct methanol fuel cell is not complicated as compared to other fuel cells. Furthermore, the miniaturization of the direct methanol fuel cell is possible. Consequently, research on the direct methanol fuel cell as a portable fuel cell is in progress.
FIG. 1 is a view typically illustrating a general direct methanol fuel cell system.
Referring to FIG. 1, the fuel cell system 10 includes: a fuel cell stack 20 having an air electrode (cathode) and a fuel electrode (anode) disposed at opposite sides of an electrolyte membrane made of a polymer material; a first pump 30 for supplying air, including oxygen, as fuel, to the cathode; a second pump 31 for supplying a methanol solution, as fuel, to the anode; a fluid tank 100 constructed such that water, carbon dioxide, and unreacted methanol generated from the fuel cell attack 20 are introduced into the fluid tank 100 through pipes 40 and 50, the carbon dioxide is discharged out of the fluid tank 100 through an outlet pipe 60, and the water and the unreacted methanol are supplied again to the fuel cell stack 20 by the second pump 31; a third pump 32 for supplying new methanol to the fuel cell stack 20 to replenish the fuel cell stack 20 with methanol having an amount corresponding to the amount of the consumed methanol; a heat exchanger 70; and a methanol tank 80.
The methanol solution supplied to the anode is separated into hydrogen ions and electrons. The hydrogen ions move to the cathode through the electrolyte membrane, and the electrons move to the cathode via an external circuit (not shown), whereby electric power is produced from the fuel cell stack 20. At this time, water is generated from the cathode, and carbon dioxide and unreacted methanol are generated from the anode. The water, the carbon dioxide, and the unreacted methanol are introduced into the fluid tank 100. Among them, the water and the unreacted methanol are mixed with pure methanol, which is supplied from the methanol tank 80 through the third pump 32 so as to replenish the fuel cell stack 20 with methanol having an amount corresponding to the amount of the consumed methanol, and the mixture is resupplied to the fuel cell stack 20.
As described above, the direct methanol fuel cell system 10 is normally operated only when a methanol solution is continuously supplied to the direct methanol fuel cell system from the outside, and carbon dioxide is continuously removed from the direct methanol fuel cell system, which is unlike chemical cells. For this reason, the function of the fluid tank, which continuously supplies the methanol solution to the direct methanol fuel cell system and continuously removes the carbon dioxide from the direct methanol fuel cell system, is very important.
Generally, the positions of pipes, through which reaction products are introduced to the fluid tank from the fuel cell stack, a pipe, through which a liquid-phase mixed solution is supplied to the fuel cell stack from the fluid tank, and a pipe, through which a gaseous carbon dioxide is discharged from the fluid tank, are fixed in the fluid tank. As a result, when the fluid tank is inclined, shaken, or turned upside down, the fuels may not be smoothly supplied to the fuel cell stack from the fluid tank, and the carbon dioxide may not be discharged from the fluid tank.
On the other hand, most of the unreacted methanol introduced into the fluid tank exists in a liquid phase. However, the unreacted methanol may be evaporated with the result that the unreacted methanol may exist in a gaseous phase. There is a great possibility that the gaseous unreacted methanol may be discharged together with the carbon dioxide. For this reason, it is very important to prevent the discharge of the unreacted methanol out of the fluid tank through appropriate collection of the unreacted methanol.
Consequently, there is high necessity for a fluid tank that is capable of effectively collecting an unreacted methanol solution discharged from a fuel cell and discharging carbon dioxide and supplying a fuel mixture to a fuel cell stack even when the fluid tank is inclined or turned upside down.